Method and system for reducing gray scale discontinuities in contrast enhancing screens affected by ambient light

ABSTRACT

A method and system for reducing a gray scale discontinuity between pixel locations in a blackened state on a contrast enhancing screen and pixel locations in a gradual shading region of an image displayed by a projector on the contrast enhancing screen. The discontinuity is caused by ambient light. The method comprises measuring an intensity of the ambient light, comparing the measured ambient light intensity to an intensity of light projected by the projector onto the gradual shading region, and generating apparent gray scale levels for the pixels to be displayed in the pixel locations in the gradual shading region based on the comparison.

BACKGROUND

Digital projection systems or devices are frequently used to display astill or video image. Viewers evaluate digital projection systems basedon many criteria such as image size, color purity, brightness,resolution, and contrast ratio. Contrast ratio, or the difference inbrightness between the lightest and darkest tones in the displayedimage, is a particularly important metric in many display markets.

One popular class of digital projection systems is a front projectionsystem. A front projection system projects an image onto a reflectivescreen which displays the image. Front projection systems areadvantageous in many different settings because of their size, cost, andease of use.

However, front projection systems are generally only suited forrelatively dark rooms because front projection screens indiscriminatelyreflect all light incident to its surface with equal efficiency. Lightfrom the projector can be diluted by light from room lights, windows,pixel to pixel interference, and/or any other ambient light. Thus,ambient light limits the effective contrast ratio of many frontprojector systems to undesirably low levels.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate various embodiments of the presentinvention and are a part of the specification. The illustratedembodiments are merely examples of the present invention and do notlimit the scope of the invention.

FIG. 1 illustrates an exemplary light engine according to one exemplaryembodiment.

FIG. 2 illustrates an exemplary spatial light modulator that may be usedin the light engine according to one exemplary embodiment.

FIG. 3 illustrates a spectrum of exemplary gray scale levels accordingto one exemplary embodiment.

FIG. 4 illustrates an exemplary video frame that has been divided into anumber of time slices according to one exemplary embodiment.

FIG. 5 illustrates an exemplary contrast enhancing screen that may beused in conjunction with the light engine according to one exemplaryembodiment.

FIG. 6 illustrates an exemplary image with a gradual shading region thatis displayed on the contrast enhancing screen according to one exemplaryembodiment.

FIG. 7 illustrates that absent the presence of ambient light, the colorof the exemplary image gradually shades from black to white according toone exemplary embodiment.

FIG. 8 illustrates that the presence of ambient light may create a sharpdiscontinuity in the color transition between the pixels that are in ablack state and pixels that are in the gradual shading region of theexemplary image according to one exemplary embodiment.

FIG. 9 is a flow chart illustrating a method of eliminating or reducingthe visual effects of the sharp discontinuity at the edge of the blackregion and the gradual shading region caused by ambient light accordingto one exemplary embodiment.

FIG. 10 illustrates an exemplary look-up table that may be used by theimage processing unit to select an appropriate dithering algorithmaccording to one exemplary embodiment.

FIG. 11 illustrates an exemplary dithering algorithm that may be used inconnection with the method described in FIG. 9 according to oneexemplary embodiment.

FIG. 12 is a flow chart illustrating that the method of eliminating orreducing the visual effects of the sharp discontinuity described inconnection with FIG. 9 may be used only if the image that is to bedisplayed includes gradual shading at the image's borders according toone exemplary embodiment.

Throughout the drawings, identical reference numbers designate similar,but not necessarily identical, elements.

DETAILED DESCRIPTION

A method and system for enhancing the contrast of an image that isprojected by a light engine onto a viewing surface while avoiding visualartifacts caused by ambient light are described herein. The light engineis configured to generate and display pixels upon the viewing surface atvarious screen pixel locations.

The viewing surface may be a contrast enhancing screen that includes anarray of screen pixel elements. Each screen pixel element may correspondto a screen pixel location on the surface of the contrast enhancingscreen. Each screen pixel element is configured to enhance the contrastof an image that is displayed on the screen by being in a black ornon-reflective state when the screen pixel element's correspondingscreen pixel location is not receiving light from the light engine. Eachscreen pixel element is also configured to be in a white or reflectivestate when the corresponding screen pixel location is receiving lightfrom the light engine. Thus, a screen pixel location corresponding to ascreen pixel element in a non-reflective state does not reflect light,including ambient light, that is directed onto the screen pixellocation. Conversely, a screen pixel location corresponding to a screenpixel element in a reflective state reflects all light, includingambient light, that is directed onto the screen pixel location.

In one exemplary embodiment, the system may include an ambient lightsensor configured to generate information indicative of the intensity ofambient light present in the vicinity of the light engine and/or theviewing surface. This information may include the intensities ofindividual components of light such as red, green, and/or blue or it mayinclude some composite luminance value. In one exemplary embodiment, theambient light sensor allows for automatic adjustment of a turn-onthreshold for the screen pixel elements so that the screen pixelelements respond only to projected light with an intensity greater thanthe intensity of the ambient light.

In one exemplary embodiment, the light engine generates and displayspixels having a gray scale level during a frame period. To generate agray scale level for a particular pixel, the light engine may deliver acertain dosage or energy of light to the pixel's corresponding pixellocation during the frame period. For a given group of pixels, the lightengine can generate a spectrum of gray scale levels from a minimumdosage to a maximum dosage.

As will be explained below, undesirable visual artifacts may be createdwhen an image having a gradual shading region is projected onto thecontrast enhancing screen in an environment where there is ambient lightpresent. For example, the contrast enhancing screen may cause a sharpcontrast or gray scale discontinuity at the border between pixellocations in a black state and pixel locations displaying the graduallyshading pixels. In one exemplary embodiment, as will be explained below,the light engine may be configured to eliminate or reduce the visualeffects of the gray scale discontinuity by using spatial and/or temporaldithering of the pixels within the gradual shading region. Spatial andtemporal dithering will be explained below.

The light engine may be configured to use the measured ambient lightintensity to estimate an “ambient light level” or, more particularly,the energy or dosage of ambient light received by a cluster or group ofpixel locations on the viewing surface during a particular frame period.In one exemplary embodiment, the light engine is configured to alter theamount of spatial and temporal dithering used to generate gray scalelevels in response to the estimated ambient light level.

More particularly, the light engine may be configured to compare theambient light energy dosage or average ambient light intensity that agroup or cluster of pixel locations receives during a frame period tothe light engine energy dosage or average light intensity that the groupof pixel locations receives from the light engine during the same frameperiod. In particular if the average ambient light intensity isapproximately equal to or greater than the light engine average lightintensity, the image processing unit may use spatial and/or temporaldithering to generate apparent gray scale levels for the pixels to bedisplayed in the group of pixel locations. In this manner, the actualdosage of light received by a given pixel location on the screen may bedominated by the influence of the light engine.

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present method and system. It will be apparent,however, to one skilled in the art that the present method and systemmay be practiced without these specific details. Reference in thespecification to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment is included in at least one embodiment. Theappearance of the phrase “in one embodiment” in various places in thespecification are not necessarily all referring to the same embodiment.

The term “light engine” will be used herein and in the appended claimsto refer to a front projector or any other display system configured todisplay an image on a screen with a contrast ratio that may be affectedby ambient light. The image may be displayed on a contrast enhancingscreen or any suitable viewing surface. The term “image” will be usedherein and in the appended claims, unless otherwise specificallydenoted, to refer broadly to a still image, series of images, motionpicture video, or anything else that is displayed by a light engine.

FIG. 1 illustrates an exemplary light engine (100) according to anexemplary embodiment. The components of FIG. 1 are exemplary only andmay be modified, changed, or added to as best serves a particularapplication. As shown in FIG. 1, image data is input into an imageprocessing unit (106). The image data defines an image that is to bedisplayed by the light engine (100). While one image is illustrated anddescribed as being processed by the image processing unit (106), it willbe understood by one skilled in the art that a plurality or series ofimages may be processed by the image processing unit (106). The imageprocessing unit (106) performs various functions including controllingthe illumination of a light source (101) and controlling a spatial lightmodulator (SLM) (103).

As shown in FIG. 1, the light source (101) may provide a beam of lightto a color device (102). The light source (101) may be, but is notlimited to, a high pressure mercury lamp. The color device (102) isoptional and enables the light engine (100) to display a color image.The color device (102) may be, but is not limited to, a sequential colordevice or a color wheel, for example.

Alternatively, the color device (102) may be a “parallel” color devicesuch as an arrangement of dichroic mirrors that split the light intoprimary colored light, such as red, green, and blue light.

Light transmitted by the color device (102) may be focused onto the SLM(103) through a lens or through some other device (not shown). An SLM isa device that modulates incident light in a spatial patterncorresponding to an electrical or optical input. The terms “SLM” and“modulator” will be used interchangeably herein to refer to a spatiallight modulator. The incident light may be modulated in its phase,intensity, polarization, direction, wavelength, color, hue, or any otherproperty inherent to light by the modulator (103). Thus, the SLM (103)of FIG. 1 modulates the light output by the color device (102) based oninput from the image processing unit (106) to form an image bearing beamof light that is eventually displayed or cast by display optics (104)onto a contrast enhancing screen (105) or other suitable viewingsurface. The display optics (104) may comprise any device configured todisplay or project an image. For example, the display optics (104) maybe, but are not limited to, a lens configured to project and focus animage onto a viewing surface. The screen (105) will be described in moredetail below.

If the color device (102) is a parallel color device, then the SLM (103)may comprise a number of modulators (103) corresponding to each primarycolor. For example, if the color device (102) outputs red, green, andblue, the light engine (100) may include three modulators (103).

As shown in FIG. 1, an ambient light sensor (107) may be coupled toscreen (105). The ambient light sensor (107) may be physically mountedon or connected to the screen (105), to a component of the light engine(100), or to something else in the room containing the screen (105).Moreover, there may be more than one (not shown) ambient light sensors(107) connected to the screen (105) or light engine (100). Ambient lightsensors (107) are known in the art and will not be explained in detail.The ambient light sensor (107) is configured to provide information tothe image processing unit (106) that is indicative of the intensity ofone or more components of the ambient light or the luminance of theambient light that surrounds or is adjacent to the screen (105) and/orthe light engine (100). The image processing unit uses this informationto estimate a dosage or energy of ambient light radiation that impingeson the screen (105). The purpose of the ambient light sensor (107) willbe further explained below.

The SLM (103) may be, but is not limited to, a liquid crystal on silicon(LCOS) array, a micromirror array, a diffractive light device (DLD), oran analog LCD panel. LCOS and micromirror arrays are known in the artand will not be explained in detail in the present specification. Anexemplary, but not exclusive, LCOS array is the Philips™ LCOS modulator.An exemplary, but not exclusive, micromirror array is the Digital LightProcessing (DLP) chip available from Texas Instruments™ Inc.

As mentioned, the SLM (103) may also be a diffractive light device(DLD), in one exemplary embodiment. A DLD has an array of SLM pixelelements or cells that are each independently controllable to receivewhite light and output light having a spectral distribution that ispeaked about a particular wavelength such as red, green, blue, cyan,magenta, yellow, violet, orange, or other colors. When we say that a SLMpixel element outputs a certain color, we mean that it is outputting aspectral distribution that is peaked about that color.

Each cell in a DLD includes an optical cavity with a dimension normal tothe array of cells that is responsive to the application of a voltageacross opposing plates that help to define the cavity. The cavity may bedefined by controlling voltage across the opposing plates or controllingcharge injection to one or both of the opposing plates. The dimension ofthat optical cavity determines the output spectral peak as discussedabove. Further, the cavity has a black state at a certain dimensionwherein nearly all of the light is absorbed.

In one alternative embodiment, the SLM (103) may be an analog LCD panelthat is configured to pass continuously varying or analog amounts ofpolarized light depending on a voltage applied to each SLM pixelelement. An LCD panel can operate in either a pulse width modulationmode or in an analog mode.

FIG. 2 illustrates an exemplary SLM (103) that may be used in theabove-described light engine (100; FIG. 1) according to one exemplaryembodiment. The exemplary SLM (103) of FIG. 2 comprises an array ofmicromirrors (120), or SLM pixel elements, for illustrative purposes.The array of micromirrors (120) comprises a number of rows ofmicromirrors (120). The micromirrors (120) may be operated in a digital,or bistable, manner. Digital operation fully deflects a givenmicromirror to either a first position or a second position. The firstposition is the “on” position and the second position is the “off”position. Light generated by the light source (101; FIG. 1) illuminatesthe entire array of micromirrors. Micromirrors deflected to the firstposition reflect light along a first path, whereas micromirrorsdeflected to a second position reflect light along a second path. Thedisplay optics (104) of the light engine collects the light from themirrors in the first or “on” position and focuses the light onto animage plane such as the screen (105; FIG. 1). The light reflected bymicromirrors in the second or “off” position is prevented from reachingthe image plane. In one exemplary embodiment, as will be explained indetail below, each micromirror or SLM pixel element may correspond to apixel location on the screen (105; FIG. 1) upon which the image isdisplayed. A pixel location on the screen (105; FIG. 1) associated withan SLM pixel element in the “on” position is illuminated, whereas apixel location on the screen (105; FIG. 1) associated with an SLM pixelelement in the “off” position is not illuminated or is in a “black”state.

FIG. 2 illustrates control circuitry (121-123) that controls theoperation of the micromirrors (120). For example, row select logic (121)and column drive logic (122) may send update data to particularmicromirrors in the array of micromirrors (120) to indicate whether themicromirrors are to be in the “on” or “off” position at a given time.Interface electronics (123) may be included in the light engine (100;FIG. 1) to interface between the other components of the light engine(100; FIG. 1) and the logic (121, 122) controlling the SLM (103). Thecontrol circuitry (121-123) is optional and may or may not be includedin the light engine (100; FIG. 1).

The SLM (103) may be configured to produce an image with varying levelsof intensity, or gray scale levels. The term “gray scale level” mayrefer to the intensity of individual primary colors, such as red, green,and blue, or it can refer to the total intensity or luminance of lightreflected off a particular screen pixel location. In one embodiment, theSLM (103) may use half-toning to generate a given intensity or grayscale level for the pixels that are displayed on the contrast enhancingscreen (105). Half-toning may be accomplished by pulse width modulationor spatial modulation, for example. In other words, a SLM pixel elementmay be rapidly turned on and off within a given frame period to generatea desired gray scale level for a pixel that is displayed in the SLMpixel element's corresponding pixel location on the screen (105; FIG.1). If an SLM pixel element is pulsed quickly enough within a givenframe, the human eye will accurately measure the gray scale level of thepixel during that frame, but will fail to detect the pulsing.Half-toning may also be accomplished by other methods such as varyingthe intensity of light delivered to the pixels' corresponding pixellocations throughout a frame period. In yet another exemplaryembodiment, half-toning may be accomplished by a combination of pulsewidth modulation and intensity variation.

FIG. 3 illustrates a spectrum of exemplary gray scale levels accordingto an exemplary embodiment. The gray scale levels of FIG. 3 areillustrative and it will be recognized that there may be more or fewerlevels of gray scale as best serves a particular light engine. As shownin FIG. 3, the first gray scale level (130) is completely black. Acompletely black gray scale level corresponds to a pixel that is in the“off” state during an entire frame (i.e. the pixel's corresponding SLMmicromirror or element is in the “off” position). As shown in FIG. 3,the gray scale levels increase in brightness until the last gray scalelevel (131). The last gray scale level (131) is white and corresponds toa pixel that is in the “on” state during an entire frame (i.e. thepixel's corresponding SLM micromirror or element is in the “on” positionduring the entire frame). The gray scale levels in between the first andlast gray scale levels (130, 131) may be generated by varying the amountof time within a given frame that the pixel is “on.”

In an alternative embodiment, if the SLM (103) is an analog device suchas an LCD panel, the SLM (103) may have an “on” state, an “off” state,and analog levels between the “on” and “off” states. An analog SLM (103)may add gray scale levels to an image by varying the analog levelbetween the “on” and “off” states.

FIG. 4 illustrates an exemplary video frame (133) that has been dividedinto a number of time slices. As will be explained below, the divisionof a video frame (133) into a number of time slices allows a lightengine (100; FIG. 1) to generate a color image with varying intensitiesor gray scale levels. Although the exemplary frame (133) of FIG. 4 isdivided into fifteen time slices, the frame (133) may be divided intoany number of time slices as best serves a particular application.

According to an exemplary embodiment, in a frame that has been dividedinto 2^(m)−1 time slices, the SLM (103; FIG. 1) may generate up to 2^(m)possible levels of gray scale for each of the pixels associated with theSLM (103; FIG. 1). In other words, the SLM (103; FIG. 1) may generate upto 2^(m) different intensities or shades of a particular color for eachof the pixels. In terms of bits, in a frame that has been divided into2^(m)−1 time slices, the SLM (103; FIG. 1) may generate up to m bits ofcolor for each of the pixels. The variable “m,” as used herein and inthe appended claims, may be any integer that is equal to or greater thanone.

The number of bits of gray scale resolution may vary as best serves aparticular application. For example, some light engines may beconfigured to generate 24-bit color, or eight bits of gray scale foreach of three primary colors. Other light engines may be configured togenerate more or less than three primary colors, each having more orless than eight bits of gray scale. Thus, an exemplary value for m maybe 24. However, as previously explained, the value of m may vary as bestserves a particular application.

FIG. 5 illustrates an exemplary screen (105) that may be used inconjunction with the light engine (100; FIG. 1) of FIG. 1. According toan exemplary embodiment, the screen (105) is a contrast enhancing screenconfigured to reflect light projected onto it by the light engine (100;FIG. 1) and thereby display an image. As shown in FIG. 5, the contrastenhancing screen (105) comprises a number of screen pixel elements(135). Each screen pixel element (135) has a corresponding screen pixellocation on the surface of the screen (105). The number of screen pixelelements (135) may vary depending on the physical dimensions of thescreen (105). Furthermore, as mentioned previously, the screen pixelelements (135) may correspond to the SLM pixel elements (120; FIG. 2) ofthe SLM (103; FIG. 2) to some extent, but they may not be perfectlyaligned or cover exactly the same screen pixel locations on the screen(105).

As its name suggests, the contrast enhancing screen (105) is configuredto enhance the contrast of an image that is displayed on the screen(105). In one exemplary embodiment, the contrast enhancing screen (105)is configured to turn screen pixel elements (135) corresponding toscreen pixel locations that do not have light projected onto them by thelight engine (100; FIG. 1) to a non-reflective state or “black state.”The contrast enhancing screen (105) is further configured to turn screenpixel elements (135) corresponding to screen pixel locations that havelight projected onto them by the light engine (100; FIG. 1) to areflective state. By turning screen pixel elements (135) that do nothave light projected onto them by the light engine (100; FIG. 1) to anon-reflective state and screen pixel elements (135) that have lightprojected onto them by the light engine (100; FIG. 1) to a reflectivestate, the screen (105) diminishes the contrast-reducing effect ofambient light that may be in the vicinity of the screen (105) andtherefore enhances the contrast of the image that is displayed on thescreen (105).

The contrast enhancing screen (105) of FIG. 5 is well-suited fordisplaying images with sharp color transitions, such as black text on awhite background for example. However, in many instances, the lightengine (105) projects an image with a gradual shading region onto thecontrast enhancing screen (105). FIG. 6 illustrates an exemplary image(140) with a gradual shading region (141) that is displayed on thecontrast enhancing screen (105).

The gradual shading region (141) shown in FIG. 6 transitions betweenblack pixel locations and white pixel locations for illustrativepurposes only. In one exemplary embodiment, the gradual shading region(141) may transition between screen pixel locations having any twocolors.

FIGS. 7 and 8 illustrate the effect of ambient light on the gradualshading region (141) of FIG. 6. First, FIG. 7 illustrates that absentthe presence of ambient light, the color of the image (140) graduallyshades (141) from black (142) to white (143). The black region (142)comprises screen pixel locations that have been turned to anon-reflective state because they do not have any light projected ontothem by the light engine (100; FIG. 1). On the other hand, the screenpixel locations in the gradual shading region (141) and in the whiteregion (143) are in a reflective state because they have light projectedonto them by the light engine (100; FIG. 1).

However, as shown in FIG. 8, the presence of ambient light may create asharp discontinuity (144) in the gray scale transition between thescreen pixel locations that are in a black state (140) and screen pixellocations that are in the gradual shading region (141). The sharpdiscontinuity (144) is due to the fact that screen pixel locations in areflective state reflect any ambient light present as well as the lightprojected by the light engine (100; FIG. 1). Accordingly, the screenpixel locations in the gradual shading region (141) reflect not only thelight shown on them by the light engine (100; FIG. 1), but also theambient light present in the vicinity of the screen (105). If the amountof ambient light is appreciable, a sharp discontinuity (144) may bevisible at the edge of the black region (140) and the gradual shadingregion (141) because of the added intensity of the pixels displayed inthe gradual shading region (141) due to the ambient light.

FIG. 9 is a flow chart illustrating a method of eliminating or reducingthe visual effects of the sharp discontinuity (144; FIG. 8) at the edgeof the black region (140; FIG. 8) and the gradual shading region (141;FIG. 8) caused by ambient light. The method comprises using a spatialand/or temporal dithering pattern to generate gray scale levels for someor all of the pixels to be displayed in the gradual shading region (141;FIG. 8). The dithering method allows the gradual shading to be generatedby an arrangement, sequence, and/or “checkerboard” pattern of “on” and“off” pixels. The dithering pattern may change from frame to frame basedon the intensity of ambient light and projected light incident upon thescreen pixel locations on the screen (105; FIG. 5).

As shown in FIG. 9, the method comprises first measuring the ambientlight intensity (step 193) present within a given frame with the ambientlight sensor (107; FIG. 1). The ambient light sensor (107; FIG. 1)communicates the ambient light intensity measurement to the imageprocessing unit (106; FIG. 1) which compares the ambient light intensityto the intensity of the light (i.e. gray scale level) in the gradualshading region (141; FIG. 6) that is projected by the light engine (100;FIG. 1) during the frame (step 194). In an alternative embodiment, thecomparison (step 194) may be performed by a component of the lightengine (100; FIG. 1) other than the image processing unit (106; FIG. 1)or by a processor unit that is not a part of the light engine (100; FIG.1). In yet another alternative embodiment, the comparison step (step194) is not performed and the dithering algorithm is based solely on themeasurement of the ambient light intensity (step 193).

To perform the comparison (step 194), the image processing unit (106;FIG. 1) may integrate the measured ambient light intensity over theframe period and convert the resulting ambient light energy density mayto a binary value. The binary value may then be compared to a binaryvalue representing the number of bits of gray scale level or intensityof the projected light (i.e. the light projected by the light engine(100; FIG. 1)) in the gradual shading region (141; FIG. 6). A binaryvalue comparison of ambient light energy density to the gray scale levelof the projected light is not necessarily the only way to perform thecomparison (step 194). Other methods for perform the comparison (step194) may be used as best serves a particular application.

After performing the comparison (step 194), the image processing unit(106; FIG. 1) selects a dithering algorithm based on the comparison(step 195). For example, in one exemplary embodiment, the imageprocessing unit (106; FIG. 1) may select the appropriate ditheringalgorithm based on the difference between the ambient light intensityand the intensity of the light projected by the light engine (100; FIG.1). The selection may be made from a look-up table, for example. In analternative embodiment, the image processing unit (106; FIG. 1) mayselect the appropriate dithering algorithm based on the total amount ofcombined ambient light intensity and the intensity of the lightprojected by the light engine (100; FIG. 1) in the gradual shadingregion (141; FIG. 6).

The particular dithering algorithm that is selected by the imageprocessing unit (106; FIG. 1) may vary as best serves a particularapplication. Moreover, in an exemplary embodiment, a number of differentdithering algorithms may be selected and used within the same gradualshading region (141; FIG. 6).

Furthermore, the number of pixels that are dithered in the gradualshading region (141; FIG. 6) may vary as best serves a particularapplication. For example, in some instances, only the pixels in thegradual shading region (141; FIG. 6) with a projected light intensityequal to or less than the ambient light intensity are dithered. In otherinstances, all of the pixels to be displayed in the gradual shadingregion (141; FIG. 6) are dithered. Yet in other instances, only pixelswith the two or three least significant bits of gray scale shading aredithered. An example of the many possible dithering algorithms will bedescribed below.

Once the dithering algorithm has been selected, the dithering algorithmis used to generate apparent gray scale levels for some or all of thepixels to be displayed in the gradual shading region (141; FIG. 6) (step196). By generating apparent gray scale levels for some or all of thepixels in the gradual shading region (141; FIG. 6), the sharpdiscontinuity (144; FIG. 8) caused by ambient light may be eliminated orits visual effects may be reduced. The term “apparent gray scale level”will be described below.

An example of the dithering method described in connection with FIG. 9will now be given. Consider a light engine (100; FIG. 1) configured togenerate eight bits of gray scale for each pixel in an image that is tobe displayed on a contrast enhancing screen (105; FIG. 1). The image inthe present example comprises a gradual shading region (141; FIG. 6).The ambient light intensity is measured by the ambient light sensor(107; FIG. 1) and is converted to a binary value corresponding to one ofthe eight bits of gray scale. In this example, the ambient light energylevel is equal to 2 bits for illustrative purposes only.

The image processing unit (106; FIG. 1) then compares the ambient lightenergy level to the intensity of the light in the gradual shading region(141; FIG. 6) that is projected by the light engine (100; FIG. 1). Basedon this comparison, the image processing unit (106; FIG. 1) selects anappropriate dithering algorithm for some or all of the pixels to bedisplayed in the gradual shading region (141; FIG. 6). FIG. 10illustrates an exemplary look-up table that may be used by the imageprocessing unit (106; FIG. 1) to select the appropriate ditheringalgorithm. As shown in FIG. 10, the image processing unit (106; FIG. 1)may select one of a number of different dithering algorithms based onthe ambient light intensity measured in bits and the projected lightintensity measured in bits. For example, if the projected lightintensity is equal to 1 bit and the ambient light intensity is equal to2 bits, then the dithering algorithm “G” may be selected.

The exemplary look-up table of FIG. 10 is simplified for illustrativepurposes. It will be recognized that the look-up table may have more orless entries than are illustrated in FIG. 10. Furthermore, theparticular dithering algorithm may be selected based on differentcriteria than are illustrated in FIG. 10.

FIG. 11 illustrates an exemplary dithering algorithm that may be used inconnection with the method described in FIG. 9. The exemplary ditheringalgorithm shown in FIG. 11 temporally and spatially dithers pixels to bedisplayed in the gradual shading region (141) such that the pixelsappear to have gray scale levels that gradually shade from 1 bit to 4bits. As shown in FIG. 11, the pixels are divided into in four pixelblocks each including four pixels. In particular, there is a first pixelblock (150), a second pixel block (151), a third pixel block (152), anda fourth pixel block (153). Each pixel block (150-153) has acorresponding pixel block location on the screen (105; FIG. 1). Afterbeing temporally and spatially dithered, the first pixel block (150) isto have an apparent gray scale level of 1 bit, the second pixel block(151) is to have an apparent gray scale level of 2 bits, the third pixelblock (152) is to have an apparent gray scale level of 3 bits, and thefourth pixel block (153) is to have an apparent gray scale level of 4bits.

As used herein and in the appended claims, unless otherwise specificallydenoted, the term “apparent gray scale level” will be used to refer toan average intensity of all the pixels within a pixel block (e.g.;150-153). The average intensity of the pixels may be calculated across anumber of frames, depending on the dithering algorithm. In general, if Xrepresents the number of pixels in a pixel block, N represents thenumber of sequential frames used by the dithering algorithm, and Yrepresents the total number of pixels in the “on” state during the Nframes, then the average intensity of the pixels is equal to Y/(N*X).For example, if a particular dithering algorithm uses two sequentialframes to accomplish the dithering of a group of four pixels and twopixels are in the “on” state during the two frames, then the averageintensity of the pixels is equal to 2/(2*4) which is 2/8. This averageintensity is equal to an apparent gray scale level of 2 bits in an 8-bitgray scale light engine according to an exemplary embodiment.

As shown in FIG. 11, the dithering algorithm temporally and spatiallydithers the pixels in each of the groups of pixels by activating, orturning “on” select pixels during one of two frames (frame M and frameM+1). As will be used herein and in the appended claims, a pixel in the“on” state is activated during a given time period and a pixel (135) inthe “off” state is not activated during the given time period in thecase of digital devices. Furthermore, the terms “turning 'on'” and“activating” a pixel herein and in the appended claims will be usedinterchangeably to refer to causing the pixel to be in the “on” state. Apixel may be turned “on” or activated by controlling the pixel'scorresponding pixel element (120; FIG. 2) within the SLM (103; FIG. 2).

As shown in FIG. 11, the first pixel block (150) is given an apparentgray scale level of 1 bit by activating only one pixel within the firstpixel block (150) during either frame M or frame M+1. Although FIG. 11depicts the top-right pixel being in the “on” state during frame M+1 andthe rest of the pixels in the first pixel block (150) being in the “off”state during both frame M and frame M+1, it will be understood that anyone of the four pixels in the first pixel block (150) may be activatedduring one of the two frames to achieve an apparent gray scale level of1 bit for the first pixel block (150).

Likewise, the second pixel block (151) is given an apparent gray scalelevel of 2 bits by activating two pixels within the second pixel block(150) during either frame M or frame M+1. As shown in FIG. 11, thebottom-left pixel is the only pixel in the second pixel block (151) thatin the “on” state during frame M and the top-right pixel is the onlypixel in the second pixel block (151) that in the “on” state duringframe M+1. In one exemplary embodiment, any two of the pixels in thesecond pixel block (151) may be activated during the two frames toachieve an apparent gray scale level of 2 bits for the second pixelblock (151).

FIG. 11 shows that the third and fourth groups of pixels (152, 153) aregiven apparent gray scale levels of 3 and 4 bits, respectively, usingthe same process described in connection with the generation of theapparent gray scale levels for the first and second groups of pixels(150, 151).

The exemplary dithering algorithm described in connection with FIG. 11is one example of the many types of dithering algorithms that may beused to eliminate or reduce the visual effects of the sharpdiscontinuity (144; FIG. 8) caused by ambient light. Other ditheringalgorithms may be used as best serves a particular application. In oneexemplary embodiment, the dithering algorithm may use any number offrames, use any number of groups of pixels, and generate any number ofapparent gray scale levels. Furthermore, there may be any number ofpixels within each of the groups of pixels.

FIG. 12 is a flow chart illustrating that the method of eliminating orreducing the visual effects of the sharp discontinuity (144; FIG. 8)described in connection with FIG. 9 may be used only if the image thatis to be displayed includes gradual shading at the image's borders. Asshown in FIG. 12, the image processing unit (106; FIG. 1) or some othercomponent of the light engine (100; FIG. 1) first determines whether theimage that is to be displayed includes edges with gradual shading (step190). If the image does include edges with gradual shading (Yes; step190), then the dithering method described in connection with FIG. 9 isused to eliminate or reduce the visual effects of the sharpdiscontinuity (144; FIG. 8) caused by the ambient light. However, if theimage does not include edges with gradual shading (No, step 190), thedithering method described in connection with FIG. 9 is not applied toany of the pixels of the image. White text on a black background is anexemplary image that does not include edges with gradual shading.

In yet another embodiment, the light engine (100; FIG. 1) may furthercomprise a user-controllable device, knob, button, or other functionconfigured to control the use of the dithering by the light engine (100;FIG. 1). For example, a user of the light engine (100; FIG. 1) maydesire that the light engine (100; FIG. 1) not use any dithering even ifthere is a significant amount of ambient light present in the room. Theuser may then set the device, knob, button, or other function such thatthe light engine (100; FIG. 1) does not use any dithering.

The preceding description has been presented only to illustrate anddescribe embodiments of invention. It is not intended to be exhaustiveor to limit the invention to any precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. It is intended that the scope of the invention be defined bythe following claims.

1. A method of reducing a gray scale discontinuity between pixellocations in a blackened state on a contrast enhancing screen and pixellocations in a gradual shading region of an image displayed by aprojector on said contrast enhancing screen, said discontinuity causedby ambient light, said method comprising: measuring an intensity of saidambient light; comparing said measured ambient light intensity to anaverage intensity of light projected by said projector onto said gradualshading region; and generating apparent gray scale levels for pixels tobe displayed in said pixel locations in said gradual shading regionbased on said comparison.
 2. The method of claim 1, further comprising:selecting a dithering algorithm based on said comparison; wherein saidstep of generating said apparent gray scale levels uses said ditheringalgorithm to generate said apparent gray scale levels for said pixels tobe displayed in said pixel locations in said gradual shading region. 3.The method of claim 1, wherein said step of generating said apparentgray scale levels for said pixels to be displayed in said pixellocations in said gradual shading region comprises: spatially andtemporally dithering pixel blocks during a number of frame periods, eachof said pixel blocks comprising a plurality of said pixels to bedisplayed in said pixel locations in said gradual shading region;wherein said spatial and temporal dithering of said pixel blocksgenerates an apparent gray scale level for each of said pixel blocks. 4.The method of claim 3, wherein, during each of said number of frameperiods, said step of spatially and temporally dithering said pixelscomprises activating one or more of said plurality of pixel locations ineach of said pixel blocks.
 5. The method of claim 3, wherein said pixelblocks each comprise four pixels.
 6. The method of claim 3, wherein saidnumber of frame periods is equal to two.
 7. A method of operating alight engine configured to project light onto a group of pixel locationsof a viewing surface during a time period, said method comprising:estimating an ambient light energy received by said group of pixellocations during said time period; determining a threshold gray scalelevel of the light engine; and dithering pixels having gray scale levelsat or below said threshold gray scale level to be displayed in saidgroup of pixel locations if said estimated ambient light energy isgreater than or substantially equal to said threshold gray scale level.8. The method of claim 7, further comprising measuring an ambient lightintensity, wherein said step of estimating said ambient light energy isbased on said measured ambient light intensity.
 9. The method of claim 7wherein said time period is one or more frame periods.
 10. The method ofclaim 7, wherein said time period is a portion of a frame period. 11.The method of claim 7, wherein said step of dithering said pixelscomprises spatially and temporally dithering pixel blocks during saidtime period, each of said pixel blocks comprising a plurality of saidpixels to be displayed in said group of pixel locations.
 12. A method ofoperating a light engine configured to generate and display an image ona viewing surface, said image formed by pixels having varying gray scalelevels, said method comprising: generating an estimate of an ambientlight intensity level; and selecting between half-toning and ditheringto generate said gray scale levels for each of said pixels in responseto said estimated ambient light level.
 13. The method of claim 12,wherein said step of generating said estimate of said ambient lightintensity level comprises measuring said ambient light intensity levelwith an ambient light sensor and transferring said measured ambientlight intensity level to said light engine.
 14. The method of claim 12,further comprising selecting a threshold gray scale level, wherein saiddithering is selected to generate said gray scale levels for each ofsaid pixels that is to have a gray scale level at or below saidthreshold gray scale level if said estimated ambient light energy isgreater than or substantially equal to said threshold gray scale level.15. The method of claim 14, wherein said half-toning is selected togenerate said gray scale levels for each of said pixels if saidestimated ambient light energy is less than said threshold gray scalelevel.
 16. The method of claim 14, wherein said dithering comprisesspatially and temporally dithering pixel blocks during a number of frameperiods, each of said pixel blocks comprising a plurality of saidpixels.
 17. A system for reducing a gray scale discontinuity betweenpixel locations in a blackened state on a contrast enhancing screen andpixel locations in a gradual shading region of an image displayed by aprojector on said contrast enhancing screen, said discontinuity causedby ambient light, said system comprising: an ambient light sensorconfigured to measure an intensity of said ambient light; an imageprocessing unit configured to compare said measured ambient lightintensity to an average intensity of light projected by said projectoronto said gradual shading region; and a spatial light modulatorconfigured to generate apparent gray scale levels for pixels to bedisplayed in said pixel locations in said gradual shading region basedon said comparison.
 18. The system of claim 17, wherein said imageprocessing unit is further configured to select a dithering algorithmbased on said comparison and said spatial light modulator is furtherconfigured to use said dithering algorithm to generate said apparentgray scale levels for said pixels to be displayed in said pixellocations in said gradual shading region.
 19. The system of claim 17,wherein said spatial light modulator is configured to generate apparentgray scale levels for said pixels to be displayed in said pixellocations in said gradual shading region by spatially and temporallydithering pixel blocks during a number of frame periods, each of saidpixel blocks comprising a plurality of said pixels to be displayed insaid pixel locations.
 20. The system of claim 19, wherein said pixelblocks each comprise four pixels.
 21. The system of claim 19, whereinsaid number of frame periods is equal to two.
 22. The system of claim17, wherein said spatial light modulator is selected from the groupconsisting of an analog based light modulator, a pulse-width modulationbased light modulator, a liquid crystal display (LCD) panel, a liquidcrystal on silicon (LCOS) device, a diffractive light device (DLD), andan array of micromirrors.
 23. A light engine for displaying an imagehaving a gradual shading region on a contrast enhancing screen, saidlight engine comprising: a spatial light modulator configured togenerate gray scale levels for pixels in said image; projector opticsconfigured to project light comprising said image onto said contrastenhancing screen, said projected light having an intensity; and anambient light sensor configured to measure an intensity of ambient lightreflecting off pixel locations in said contrast enhancing screencorresponding to said gradual shading region; wherein said spatial lightmodulator reduces a gray scale discontinuity caused by said ambientlight between pixel locations in a blackened state on said contrastenhancing screen and said pixel locations in said gradual shading regionby generating apparent gray scale levels for said pixels to be displayedin said pixel locations in said gradual shading region based on acomparison between said measured ambient light intensity and saidprojected light intensity.
 24. The system of claim 23, wherein saidlight engine further comprises: an image processing unit configured toselect a dithering algorithm based on said comparison; wherein saidspatial light modulator is further configured to use said ditheringalgorithm to generate said gray scale levels for said pixels to bedisplayed in said pixel locations in said gradual shading region. 25.The system of claim 24, wherein said dithering algorithm comprisesspatially and temporally dithering pixel blocks during a number of frameperiods, each of said pixel blocks comprising a plurality of said pixelsto be displayed in said pixel locations in said gradual shading region.26. The system of claim 25, wherein said number of frame periods isequal to two.
 27. The system of claim 24, wherein said spatial lightmodulator is selected from the group consisting of an analog based lightmodulator, a pulse-width modulation based light modulator, a liquidcrystal display (LCD) panel, a liquid crystal on silicon (LCOS) device,a diffractive light device (DLD), and an array of micromirrors.
 28. Aprojector system for displaying an image on a viewing surface, saidsystem comprising: a light engine configured to generate pixels havinggray scale levels to be displayed in corresponding pixel locations onsaid viewing surface; and an ambient light sensor configured to measurean intensity of ambient light reflecting off said pixel locations onsaid viewing surface; wherein said light engine is further configured toreceive said measured ambient light intensity from said ambient lightsensor and select between a half-toning algorithm and a ditheringalgorithm to generate said gray scale levels for each of said pixelsbased on said measured ambient light intensity.
 29. The system of claim28, wherein said dithering algorithm is selected to generate said grayscale levels for each of said pixels that is to have a gray scale levelat or below a predetermined threshold gray scale level if said estimatedambient light energy is greater than or substantially equal to saidthreshold gray scale level.
 30. The system of claim 29, wherein saidhalf-toning algorithm is selected to generate said gray scale levels foreach of said pixels if said estimated ambient light energy is less thansaid threshold gray scale level.
 31. The system of claim 28, whereinsaid dithering algorithm comprises spatially and temporally ditheringpixel blocks during a number of frame periods, each of said pixel blockscomprising a plurality of said pixels to be displayed in said pixellocations in said gradual shading region.
 32. The system of claim 31,wherein said number of frame periods is equal to two.
 33. The system ofclaim 28, wherein said light engine comprises a spatial light modulatorconfigured to generate said gray scale levels of said pixels.
 34. Thesystem of claim 33, wherein said spatial light modulator is selectedfrom the group consisting of an analog based light modulator, apulse-width modulation based light modulator, a liquid crystal display(LCD) panel, a liquid crystal on silicon (LCOS) device, a diffractivelight device (DLD), and an array of micromirrors.
 35. The system ofclaim 28, wherein said viewing surface comprises a contrast enhancingscreen.
 36. A system for reducing a gray scale discontinuity betweenpixel locations in a blackened state on a contrast enhancing screen andpixel locations in a gradual shading region of an image displayed by aprojector on said contrast enhancing screen, said discontinuity causedby ambient light, said system comprising: means for measuring anintensity of said ambient light; means for comparing said measuredambient light intensity to an average intensity of light projected bysaid projector onto said gradual shading region; and means forgenerating apparent gray scale levels for pixels to be displayed in saidpixel locations in said gradual shading region based on said comparison.37. The system of claim 36, further comprising: means for selecting adithering algorithm based on said comparison; wherein said means forgenerating said apparent gray scale levels uses said dithering algorithmto generate said apparent gray scale levels for said pixels to bedisplayed in said pixel locations in said gradual shading region. 38.The system of claim 36, wherein said means for generating said apparentgray scale levels for said pixels to be displayed in said pixellocations in said gradual shading region comprises: means for spatiallyand temporally dithering pixel blocks during a number of frame periods,each of said pixel blocks comprising a plurality of said pixels to bedisplayed in said pixel locations in said gradual shading region;wherein said means for spatial and temporal dithering of said pixelblocks generates an apparent gray scale level for each of said pixelblocks.
 39. The system of claim 38, wherein, during each of said numberof frame periods, said means for spatially and temporally dithering saidpixels comprises means for activating one or more of said plurality ofpixel locations in each of said pixel blocks.
 40. A system for operatinga light engine configured to project light onto a group of pixellocations of a viewing surface during a time period, said systemcomprising: means for estimating an ambient light energy received bysaid group of pixel locations during said time period; means fordetermining a threshold gray scale level of said light engine; aridmeans for dithering pixels having gray scale levels at or below saidthreshold gray scale level to be displayed in said group of pixellocations if said estimated ambient light energy is greater than orsubstantially equal to said threshold gray scale level.
 41. The systemof claim 40, further comprising means for measuring an ambient lightintensity, wherein said means for estimating said ambient light energyis based on said measured ambient light intensity.
 42. A system foroperating a light engine configured to generate and display an image ona viewing surface, said image formed by pixels having varying gray scalelevels, said system comprising: means for generating an estimate of anambient light intensity level; and means for selecting between ahalf-toning means and a dithering means to generate said gray scalelevels for each of said pixels in response to said estimated ambientlight level.
 43. The system of claim 42, wherein said means forgenerating said estimate of said ambient light intensity level comprisesmeans for measuring said ambient light intensity level.
 44. The systemof claim 42, further comprising means for selecting a threshold grayscale level, wherein said dithering means is selected to generate saidgray scale levels for each of said pixels that is to have a gray scalelevel at or below said threshold gray scale level if said estimatedambient light energy is greater than or substantially equal to saidthreshold gray scale level.
 45. The system of claim 44, wherein saidhalf-toning means is selected to generate said gray scale levels foreach of said pixels if said estimated ambient light energy is less thansaid threshold gray scale level.
 46. The system of claim 44, whereinsaid dithering means comprises spatially and temporally dithering pixelblocks during a number of frame periods, each of said pixel blockscomprising a plurality of said pixels.